(CNN) -- A North Korean patrol boat seized a South Korean fishing boat with four people onboard Thursday morning after it strayed into North Korean waters, the South Korean news agency Yonhap cited officials as saying.

The 29-ton vessel was returning from its fishing operations and mistakenly crossed seven miles (11 km) into North Korean waters, South Korea's Defense and Unification Ministry said.

The weather was good. Ministry officials said they believed the incident, which occurred shortly after 6 a.m. (9:30 p.m. GMT Wednesday), stemmed from a faulty GPS system on the vessel.

"We are pushing for its release," Yonhap cited the official as saying, adding North Korea had yet to respond to South Korea's call to immediately return the ship "for humanitarian reasons."

The fishing boat was scheduled to return to its port of Geojin on the eastern coast Friday morning, the official said, according to Yonhap.

"There were two recent incidents in 2005 and 2006 in which South Korean vessels crossed over into the North, and records show they were returned to South after three and 18 days, respectively," a ministry official said.